1. Field of the Invention
The present invention relates to a medical transceiver device and a method, respectively, for radio-based communication with an implantable medical device, and to a system comprising the medical transceiver device and the implantable medical device.
2. Description of the Prior Art
In the context of implantable medical devices, it has become common to provide a communication link between the implanted device and an external programmer or monitor in order to allow for transmission of commands from the external device to the implanted device and to allow for transmission of stored information and/or sensed physiological parameters from the implanted device to the external programmer. Conventionally, communication between an implanted device and an external programmer has been accomplished by means of a telemetry system, which includes a transceiver located within the implanted medical device and an external programmer or monitor, also having a transceiver and one or more antennas.
U.S. Pat. Nos. 6,167,312 and 6,169,925 disclose such medical communication systems, the contents of which being incorporated herein by reference.
The former patent describes a device for use in communication with an implantable medical device, which is provided with a spatial diversity antenna array mounted to the device housing and an operating at defined frequency, located within the device housing, coupled to the antenna array. The antenna array includes at least one antenna permanently mounted to the device housing and an antenna removably mounted to the device housing and locatable at a distance from the housing, coupled to the radio frequency transceiver. A device controller selects which of the antennas in the antenna array is coupled to the transceiver responsive to the quality of signals received by the antennas.
The latter patent describes also a spatial diversity antenna array. Here, the antenna array comprises two antennas spaced a fraction of the wavelength of the defined frequency from one another, each antenna including two antenna elements mounted to the housing and located orthogonal to one another. Selection of which of the antennas is employed is accomplished by a device controller, responsive to the quality of the signals received by the antennas.
The patents described above thus address the problem of different signal strengths in space.
Another problem when transferring signals between two antennas is, however, that the receiving antenna will only sense maximum signal strength if its polarization state is similar to the polarization state of the incoming signal. In the case of linearly polarized waves, a simple way to achieve this is to simply rotate the receiving antenna until it is aligned with the field vector of the incoming linearly polarized waves.
A particular problem when transferring radio-based signals between an implantable medical device and an external device is due to the fact that the antenna polarization of the implantable medical device will be arbitrary. Since the antenna of the implantable medical device for several reasons needs to be a small and simple antenna, any system optimization with respect to polarization has to be made to the external communication device.
Neither U.S. Pat. No. 6,167,312 nor U.S. Pat. No. 6,169,925 disclose an external communication device provided with polarization diversity functionality. Even if their antennas would be maximally sensitive to radio frequency signals of different polarization, they are located far from each other, and low received signal strength could depend on position rather than polarization sensitivity of the receiving antenna. The patents provide no solution for determining an optimum polarization state for radio frequency signals to be transmitted to the implantable medical device and/or to be received from there.